The present invention is adapted for the construction of improved masks to be used in lithographic transfer of patterns which is itself a process widely employed in the production of integrated circuits and the like. Lithographic transfer of patterns has been developed because integrated circuits and the like require patterns with extremely small dimensions and fine tolerances. All forms of lithographic pattern transfer require a source of radiation and a mask to modulate this source of radiation so as to transfer a desired pattern to a receiver which is coated with a material responsive to the radiation. One particular type of radiation that has been found extremely suitable for lithographic pattern transfer is x-ray illumination. However, a problem has existed in employing x-ray illumination in that it requires careful selection of masking materials as well as relatively high aspect ratios for the masks.
More particularly, it should be apparent to those skilled in the art that a figure of merit for any particular type of mask is the contrast ratio attainable with the mask. That is, the mask is comprised of materials either transmissive, or absorbtive, of the illuminating radiation. The contrast ratio is a figure of merit defining the effectiveness of the mask in absorbing radiation that is not intended for transmission, and at the same time, transmitting the radiation that is intended to be transmitted. One factor which is extremely significant in determining the contrast ratio of any mask is the aspect ratio; the ratio between the height of the masking material versus the width of that material.
Because of the prior art methods of mask manufacturing, a rule of thumb has evolved which has limited the aspect ratio to be less than or equal to one. That is, it has been considered difficult to fabricate masks in which the thickness of the masking material was greater than the width of the material. This has severly hampered the art in reducing the dimensions of components of various integrated circuits for the reason that the final component dimension is proportional to the width of the mask producing it. For instance, platinum and copper have to be at least 0.3 microns thick to give a contrast ratio which is greater than about 3 to 1 for X-rays of a wavelength of 8.3A. Since this contrast ratio is on the borderline for reasonable resist processing such masks have required widths of at least 0.3 microns.
This limitation on the aspect ratio is a function of the prior art method of fabricating masks. In the prior art fine patterned masks have been fabricated employing electron beam illumination. Because of electron scattering, which increases with the thickness of the resist material the previously mentioned rule of thumb has been adopted. In particular a resist is coated on a substrate and illuminated by an electron beam modulated by a suitable mask defining a particular pattern. Most practical systems scan the electron beam across the areas to be exposed and control the intensity of the beam by computer control. The exposed resist may be removed, by developing, and metal or the like plated in the areas vacated by the removed resist. To the extent that the removed resist, and hence the plated metal, is proportional in shape to the mask, the pattern has been effectively transferred. In an effort to increase the thickness of the fabricated mask the resist thickness has been increased. However, electron beam scattering in this thicker resist causes the exposed resist to vary in shape from the mask shape. Thus pattern transfer is degraded by resist thickness greater than about 1-2 microns. Of course, employing prior art techniques, it is still possible to build up thick masks, with reduced effects of electron beam scattering by fabricating the mask in a number of stages. That is, employing prior art techniques, (i.e., relatively thin resists) illuminating the mask to be manufactured by an electron beam modulated by a master mask, an intermediate product is produced. This product is then coated with an additional layer of resist material and a second exposure is effected employing the same master mask and the same electron beam illumination. Of course, any number of exposure steps could be employed to thereby build up the thickness of the mask. The significant drawback to this technique is the problems inherent in registering the master mask with the mask that is being produced in a number of different exposure steps. Any misregistration will result in a degraded product.
It is therefore an object of the present invention to provide a method of constructing a mask having aspect ratios greater than one. It is another object of the present invention to provide an improved method of constructing a mask which is capable of producing a mask having aspect ratios of at least 2 to 1.
It is another object of the present invention to provide a process capable of producing masks with high aspect ratios and at the same time eliminating the alignment or registration problems inherent in multiple exposure techniques.